Lelwel generates recursive descent parsers for Rust using LL(1) grammars.
Conflicts are resolved with semantic predicates.
Semantic actions are used for ad hoc syntax-directed translation.
A special error rule can be used to recover from syntax errors.
Lelwel is written as a library, which is used by the CLI tool llw and the language server lelwel-ls.
The parser for lelwel grammar files (*.llw) is itself generated by lelwel.
There are plugins for Neovim and Visual Studio Code that use the language server.
The following examples show grammars for a basic calculator and JSON.
```antlr // token definitions
token Num{f64}='number'; token Add='+' Sub='-' Mul='*' Div='/'; token LPar='(' RPar=')';
// grammar rules
start{f64}: expr #1 ; expr{f64}: term #1 ( '+' term #2 | '-' term #3 )* #4 ; term{f64}: atomic #1 ( '' atomic #2 | '/' atomic #3 ) #4 ; atomic{f64}: Num #1 | '(' expr ')' #2 ;
// semantic actions
start#1 { Ok(expr) }
expr#1 { let mut res = term; } expr#2 { res = res + term; } expr#3 { res = res - term; } expr#4 { Ok(res) }
term#1 { let mut res = atomic; } term#2 { res = res * atomic; } term#3 { res = res / atomic; } term#4 { Ok(res) }
atomic#1 { Ok(Num.0) } atomic#2 { Ok(expr) } ```
```antlr
token True='true' False='false' Null='null';
token LBrace='{' RBrace='}' LBrak='[' RBrak=']' Comma=',' Colon=':';
token String{String}='
start{Value}: value #1 ; value{Value}: object #1 | array #2 | String #3 | Number #4 | 'true' #5 | 'false' #6 | 'null' #7 ; object{Value}: '{' (member #1 (',' member #2)* | error #3)? '}' #4 ; member{(String, Value)}: String ':' value #1 ; array{Value}: '[' (value #1 (',' value #2)* | error #3)? ']' #4 ;
limit 1000;
preamble { use std::collections::BTreeMap; use super::diag::*; use super::Value; } parameters { diag: &mut Diag } error { Code }
start#1 { Ok(value) }
value#1 { Ok(object) } value#2 { Ok(array) } value#3 { Ok(Value::String(String.0)) } value#4 { Ok(Value::Number(Number.0)) } value#5 { Ok(Value::Bool(true)) } value#6 { Ok(Value::Bool(false)) } value#7 { Ok(Value::Null) }
object#0 { let mut members = BTreeMap::new(); } object#1 { members.insert(member.0, member.1); } object#2 { members.insert(member.0, member.1); } object#3 { diag.error(errorcode, errorrange); } object#4 { Ok(Value::Object(members)) }
member#1 { Ok((String.0, value)) }
array#0 { let mut values = vec![]; } array#1 { values.push(value); } array#2 { values.push(value); } array#3 { diag.error(errorcode, errorrange); } array#4 { Ok(Value::Array(values)) } ```
Run cargo install --features="cli","lsp" lelwel.
Lelwel and its generated code is licensed under either of
at your option.
Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.